(a) Field of the Invention
This invention relates to focus detecting devices adapted to such optical instruments as cameras, microscopes, hard endoscopes and retinal cameras and medical instruments, and more particularly to a focus detecting device wherein the position of the object image is displaced by using light beams passing through different courses in an optical system so as to thereby detect the position of a focus.
(b) Description of the Prior Art
There are various focus detecting devices of this kind. For example, in a system of producing a phase difference of an image by dividing the pupil as mentioned in U.S. Pat. appln. Ser. Nos. 372,374, U.S. Pat. No. 4,521,108; 511,206 now abandoned; 582,734, now abandoned and 582,736, now U.S. Pat. No. 4,633,073 it has been necessary to provide a mechanical means for dividing the pupil and such electric means as a liquid crystal near the pupil position in the optical system and therefore there has been a problem that the optical system is complicated. Also, in a system of seeing a part of the pupil with a lens in order to avoid arranging the pupil dividing means in the optical system as mentioned in Japanese Patent laid-open No. 126221/1980, a plurality of light receiving elements detecting the displacement of the image are required and there has been a problem that the adjustment is difficult. Further, in a system wherein a small lens is arranged on the image surface and a combination of elements for seeing different parts of the pupil is provided for each picture element as mentioned in Japanese Patent laid-open No. 130506/1980, its own complicated light receiving element is required and there has been a problem that the signal processing system is complicated. In the above mentioned systems utilizing the light from the object, there has been a problem that, in case the object is dark, it will be impossible to detect a focus. On the other hand, a system of projecting a light toward an object is suggested but is a range finder system and therefore is not adapted to single lens reflex cameras and microscopes which are preferably of the TTL system.